1. Field of the Invention
The present invention relates to an optical scanner using a laser beam.
2. Description of the Related Art
For example, an optical scanning system is shown in Japanese Patent Publication (KOKOKU) No. 60-642, Japanese Patent Application Laying Open (KOKAI) No. 1-302217, etc. In this optical scanning system, a laser beam is emitted from a light source device having a laser diode (LD) light source and is reflected on a deflecting-reflecting face as a deflected light beam. The deflected light beam is then converged by a scanning image forming optical system as a light spot onto a scanned face so as to perform an optical scanning operation. This optical scanning system is widely known in association with an optical printer, etc.
A diameter of the light spot must be stabilized to perform the optical scanning operation with high accuracy in a high density record, etc. For example, the diameter of the light spot becomes unstable by an aperture arranged between the light source device and an optical deflector.
The aperture is originally arranged to adjust a shape of the light spot and the light spot diameter and remove stray light, etc. The aperture interrupts a peripheral portion of the laser beam from the light source device. A light intensity distribution of the laser beam transmitted through the aperture is formed by this interruption of the peripheral portion of the laser beam in a shape in which a base portion of an original Gaussian type light intensity distribution of the laser beam is cut. Accordingly, an influence of diffraction is caused by the aperture in the intensity distribution of the light spot converted onto the scanned face.
In particular, in the optical scanner shown in each of the above Japanese publications, a real image of the aperture can be formed in the vicinity of the scanned face in a certain aperture position by an optical system including the scanning image forming optical system and arranged after the aperture when an image forming magnification of the scanning image forming optical system is smaller than one in a cross scan-corresponding direction. In this case, the cross scan-corresponding direction is set to a direction corresponding and parallel to a cross scanning direction on a virtual optical path provided by linearly developing an optical path from the light source to the scanned face along an optical axis of the scanning image forming optical system. In the following description, a main scan-corresponding direction is set to a direction corresponding and parallel to a main scanning direction on this virtual optical path. The diameter of the light spot is greatly changed by the diffractive influence in accordance with a shift between an image forming position of the deflected light beam and the scanned face. Namely, the diameter of the light spot is greatly changed by the diffractlye influence in accordance with a defocusing amount. Therefore, no stable diameter of the light spot can be obtained.
Japanese Patent Application Laying Open (KOKAI) No. 4-229819 shows an optical scanner for forming the real image of an aperture on this side with respect to the scanned face. However, there is a problem that the diameter of a light spot is greatly changed by an influence of Fresnel diffraction in accordance with the above defocusing amount.